To get Modbus TCP/IP comms established, I downgraded to Visilogic version 9.8.65 with OS 4.004(31).
I could not get 9.8.95 with OS 4.10(48) to work.
To get the TCP connection to close on inactivity, setting SI-105 (Socket 2 TCP/IP Keep Alive) to 10, seem to be the fix.